Method of stretching film and such film

ABSTRACT

A method of stretching films in which all or a portion of the width of the film is cooled during or just after stretching so as to improve the uniformity of the film. The includes stretching a polymeric film in a tenter that grasps the film with a plurality of clips along the opposing edges of the film and propels the clips to thereby stretch the film. The tenter includes driven clips and idler clips, with at least one idler clip between respective pairs of driven clips. The cooling is done so as to improve the uniformity of the clip spacing relative to the spacing obtained at otherwise identical process conditions without such cooling.

This application is a divisional of U.S. Ser. No.09/469,972, filed Dec.21, 1999, now allowed, the disclosure of which is herein incorporated byreference.

TECHNICAL FIELD

The present invention generally relates to methods of stretching filmsand to the resulting films, and more particularly to methods ofstretching films in which all or a portion of the width of the film iscooled during or just after stretching so as to improve the uniformityof the film and to the resulting films.

BACKGROUND OF THE INVENTION

It has been known in the art to biaxially stretch films. Additionally,several methods and apparatuses have been described for biaxiallystretching films simultaneously in two directions. See, e.g., U.S. Pat.Nos. 2,618,012; 3,046,599; 3,502,766; 3,890,421; 4,330,499; 4,525,317;and 4,853,602.

Tenters have been used for the transverse direction stretching insequential biaxial film stretching processes. For a simultaneous biaxialstretching process, tenter stretching is performed on a tenter apparatusthat has grips or clippers that grasp the film along the opposing edgesof the film and propels the grasping means at varying speeds alongguiding means, which typically are rails. As used herein, “grippers” and“clips” include other film-edge grasping means, and the word “rails”includes other clip guide means. By increasing clip speed in the machinedirection, stretching in the machine direction occurs. By using suchmeans as diverging rails, transverse direction stretching occurs. Suchstretching can be accomplished, for example, by the methods andapparatus disclosed in U.S. Pat. Nos. 4,330,499 and 4,595,738, in whicheach of the clips is mechanically driven in the tenter apparatus. Morerecently, tenter frames for stretching films have been described inwhich the clips that propel the film through the tenter apparatus aredriven by linear motors. See, e.g., the methods and tenter apparatusdisclosed in U.S. Pat. Nos. 4,675,582; 4,825,111; 4,853,602; 5,036,262;5,051,225; and 5,072,493.

In the simultaneous biaxial stretching apparatus described in U.S. Pat.No. 5,051,225, tenter clips are driven by linear electric motors. Forreasons of spacing and cost, tenters such as described in the '225patent may not have every clip driven by a linear motor. For example,every third clip on each rail may be driven by a linear motor with theintervening two clips being nondriven, and thus propelled forward onlyby the film itself. Such nondriven clips are referred to as idler clips.It has been observed that the relative position of the idler clips tothe driven clips is not necessarily the ideal position of being equallyspaced between driven clips. Any inequality in the clip-to-clip spacingamong two nearest-neighbor driven clips on a rail and their interveningidler clips may be referred to using such terms as idler non-uniformity,uneven clip spacing, non-uniform clip spacing, and the like. Two specialcases, however, are important. The case in which the first and last (oronly) idler clip(s) between a pair of driven clips on a rail arepropelled forward by the film in an amount less than would be necessaryfor equal spacing among clips is referred to as idler lag or lagging.The case in which the first and last (or only) idler clip(s) between apair of driven clips on a rail are propelled forward by the film in anamount greater than would be necessary for equal spacing among clips isreferred to as idler lead or leading. In the case where there is morethan one idler clip between each pair of driven clips on each rail, itis possible to have one propelled forward by the film in an amount lessthan would be necessary for equal spacing among clips and,simultaneously, to have the other propelled forward by the film in anamount greater than would be necessary for equal spacing among clips.This situation results in an uneven clip spacing, or idlernon-uniformity, which is neither an idler lag nor an idler lead.

U.S. Pat. No. 5,753,172 describes a process for the simultaneous biaxialstretching in a tenter frame of a thermoplastic polymer film havingbeaded edges, comprising gripping the beaded edges of the film withtenter clips and increasing the temperature of the beaded edges towithin the film orientation temperature range prior to or duringsimultaneous stretching, and in subsequent stretching or heat-settingsteps, by focusing heat on the beaded edges of the film. The '172 patentstates that bead temperatures that are either too high or too low orbeads that are too thin can cause the spacing of the idler clips to benon-uniform. Column 3, lines 30-33; column 11, lines 58-62. The '172patent further states that it is generally desirable for the temperatureof the beads to be approximately equal to, or higher than, thetemperature of the central film web. Column 5, lines 27-29. The '172patent also states that the need for separate control of beadtemperatures is driven in part by the unequal heating applied to thebeads compared to the film in typical stretcher heating zones. Col. 5,lines 33-35. It is both well-known in the art and demonstrated in the'172 patent (Col. 11, lines 35-40) that such unequal heating in typicalstretcher heating zones leads to the beads being cooler than the centralfilm web. U.S. Pat. Nos. 3,231,642; 3,510,552; and 5,429,785 alsodiscuss certain effects of temperature control in various filmstretching processes.

SUMMARY OF THE INVENTION

The present inventors have discovered that by cooling all or a portionof the width of the film by an effective amount during and/or just afterstretching, clip spacing non-uniformity, particularly idler cliplagging, can be minimized to provide more uniformly spaced idler clips,and to provide a final film with more uniform properties andcharacteristics. Cooling can also be used to cause idler clip leading,if desired.

In the simultaneous biaxial stretching apparatus of the type describedin the '225 patent discussed above, tenter clips are driven by linearelectric motors. For spacing or cost reasons, not every clip is drivenby a linear motor. For example, every second or every third clip on eachrail may be driven with the intervening idler clip(s) being nondriven,and thus propelled forward only by the film itself. The relativeposition of the idler clips to the driven clips is a complex result ofthe interactions of film and process variables, such as the film'svisco-elastic properties (e.g. stress as a function of strain ratehistory) and caliper profile, and the stretching and temperatureprofiles as functions of position along the tenter. Idler clips arepropelled forward through the tenter by force imparted by the drivenclip in front of the idler(s) and the film material between them. At thesame time, the forward motion of each idler clip may be resisted byforce imparted by the driven clip and film material behind it. As thefilm is stretched in the machine and transverse directions downweb, acomplex interaction among the film material, the idler and driven clips,and the bearing frictions within the clips usually results in a netbackward force on an idler clip, when viewed in a frame of referencewhich is moving with the forward driven clip. Since there is no linearmotor force on the idler clips to counter this force, the idler clipslag behind their ideal positions. At the exit end of the tenter, wherethe film has been cooled, the idler lagging may be accompanied bypermanent downweb variations in machine direction draw ratio that extendacross the width of the film. Idler clip lagging is a result ofprocessing conditions which also adversely affect the uniformity of thefilm properties such as caliper, mechanical properties, and opticalproperties. Idler clip lagging occurs at different locations in theprocess and to greater or lesser extents depending upon the material andthe stretching conditions. Thus, it would be most advantageous tocontrol the clip lagging throughout the process (lag history), though webelieve there will be considerable advantage to controlling themagnitude of the overall, or final, clip lagging.

The present invention provides methods to reduce clip lagging to causethe idler clips to be closer to or at their ideal positions relative toadjacent driven clips, and in some cases to reverse clip lagging,causing the idler clips to be in front of their ideal positions (idlerlead). One method is edge cooling. In edge cooling, the edge portions ofthe film are cooled an effective amount at effective locations in thestretch section of the tenter and/or in the section immediately afterthe stretch section, referred to herein as the post-stretch treatmentsection. Edge cooling is believed to increase the modulus of elasticityof the material at the edge portions in a controlled fashion so that anidler clip is pulled forward more than would be the case without edgecooling by the driven clip and stiffer (cooler) edge bead in front ofit, resulting in a decrease in clip lagging. As a result, the idler cliplagging is reduced, eliminated, or reversed (idler lead). A secondmethod is zone cooling, in which substantially the entire width of theweb is cooled an effective amount at effective locations, or zones, inthe stretch section of the tenter and/or in the post-stretch treatmentsection. Zone cooling is believed to increase the modulus of elasticityof the film across substantially the entire width of the web in acontrolled fashion, so that an idler clip is pulled forward by thedriven clip and film in front of it more than would be the case withoutzone cooling, resulting in a decrease of the backward force that causesclip lagging without zone cooling.

One aspect of the present invention provides an improvement to themethod of stretching a polymeric film comprising the steps of graspingthe film with a plurality of clips along the opposing edges of the filmand propelling the clips to thereby stretch the film. The plurality ofclips includes driven clips and idler clips, with at least one idlerclip between respective pairs of driven clips. The improvement comprisesheating the polymeric film to a sufficiently high temperature to allow asignificant amount of stretching without breaking, and activelyimparting a machine direction cooling gradient to at least a portion ofthe width of the stretched film in an effective amount to improve theuniformity of spacing of the driven and idler clips.

In another aspect, the present invention provides an improvement to themethod of stretching a polymeric film comprising the steps of graspingthe film with a plurality of clips along the opposing edges of the filmand propelling the clips to thereby stretch the film. The plurality ofclips includes driven clips and idler clips, with at least one idlerclip between respective pairs of driven clips. The improvement comprisesheating the center portion and edge portions of the polymeric film to asufficiently high temperature to allow a significant amount ofstretching without breaking, maintaining, at the onset of stretching,the edge portions of the film no hotter than the center portion of thefilm, and imparting a machine direction cooling gradient to at least aportion of the width of the stretched film in an effective amount toimprove the uniformity of spacing of the driven and idler clips.

In one preferred embodiment of the above method, maintaining the edgeportions of the film no hotter than the center portion of the filmincludes actively cooling the opposed edge portions of the film.

In still another aspect, the present invention provides an improvementto the method of stretching a polymeric film comprising the steps ofgrasping the film with a plurality of clips along the opposing edges ofthe film and propelling the clips to thereby stretch the film. Theplurality of clips includes driven clips and idler clips, with at leastone idler clip between respective pairs of driven clips. The improvementcomprises heating the polymeric film to a sufficiently high temperatureto allow a significant amount of stretching without breaking, andimparting a machine direction cooling gradient to at least a portion ofthe width of the stretched film in an effective amount to reduce thevalue of idler clip lag from the value of idler clip lag in the absenceof said cooling.

In yet another aspect, the present invention provides an improvement tothe method of stretching a polymeric film comprising the steps ofgrasping the film with a plurality of clips along the opposing edges ofthe film and propelling the clips to thereby stretch the film. Theplurality of clips includes driven clips and idler clips, with at leastone idler clip between respective pairs of driven clips. The improvementcomprises heating the polymeric film to a sufficiently high temperatureto allow a significant amount of stretching without breaking, andimparting a cooling gradient to at least a portion of the width of thestretched film in an effective amount to improve the downweb caliperuniformity relative to the downweb caliper uniformity in the absence ofsaid cooling.

In still another aspect, the present invention provides an improvementto the method of stretching a pre-crystallized polymeric film comprisingthe steps of grasping the film with a plurality of clips along theopposing edges of the film and propelling the clips to thereby stretchthe film. The plurality of clips includes driven clips and idler clips,with at least one idler clip between respective pairs of driven clips.The improvement comprises heating the polymeric film to a sufficientlyhigh temperature to allow a significant amount of stretching withoutbreaking, and imparting a cooling gradient to at least a portion of thewidth of the stretched film in an effective amount to improve theuniformity of spacing of the driven and idler clips.

In yet another aspect, the present invention provides an improvement tothe method of stretching a vinyl polymer film comprising the steps ofgrasping the film with a plurality of clips along the opposing edges ofthe film and propelling the clips to thereby stretch the film. Theplurality of clips includes driven clips and idler clips, with at leastone idler clip between respective pairs of driven clips. The improvementcomprises heating the polymeric film to a sufficiently high temperatureto allow a significant amount of stretching without breaking, andimparting a cooling gradient to at least a portion of the width of thestretched film in an effective amount to improve the uniformity ofspacing of the driven and idler clips.

In one preferred embodiment of the any of the above methods, the opposededge portions of the film are cooled.

In another preferred embodiment of any of the above methods, the centerportion of the film is cooled.

In another preferred embodiment of any of the above methods,substantially the entire width of the film is cooled.

In another preferred embodiment of any of the above methods, at least aportion of the film is cooled by at least 3° C.

In another preferred embodiment of any of the above methods, the clipsare propelled through a stretch section in which the film is stretchedand subsequently through a post-stretch treatment section, and thecooling is performed in at least one of the stretch section and thetreatment section.

In another preferred embodiment of any of the above methods, the film isbiaxially stretched. More preferably, the film is simultaneouslybiaxially stretched by propelling the clips at varying speeds in themachine direction along clip guide means that diverge in the transversedirection. Still more preferably, the film is stretched to a finalstretch ratio of at least 2:1 in the machine direction and at least 2:1in the transverse direction.

In another preferred embodiment of any of the above methods, there areat least two idler clips between each respective pair of driven clips.

In another preferred embodiment of any of the above methods, the filmcomprises a thermoplastic film. More preferably, the film comprises asemi-crystalline film. Of the semi-crystalline embodiments, onepreferred film has a degree of crystallinity greater than about 1% priorto the heating. Still more preferably, the degree of crystallinity isgreater than about 7% prior to the heating. Still more preferably, thedegree of crystallinity is greater than about 30% prior to the heating.

In another preferred embodiment of any of the first four or the sixth ofthe above methods, the film comprises a thermoplastic film which is anamorphous film.

In another preferred embodiment of any of the above methods, the filmcomprises a vinyl polymer. More preferably, the film comprises apolyolefin. Still more preferably, the film comprises polyethylene orpolypropylene.

In another preferred embodiment of any of the above methods, the filmcomprises polypropylene, and the film is stretched to a final areastretch ratio of at least 16:1. More preferably, the film is stretchedto a final area stretch ratio of from 25:1 to 100:1.

In another preferred embodiment of any of the above methods, the filmcomprises polypropylene, and the film is heated to from 120 to 165° C.More preferably, the film is heated to from 150 to 165° C.

In another preferred embodiment of any of the above methods, the filmcomprises polypropylene, the film is heated to from 120 to 165° C., andthe cooling includes forcing cooling air onto the film. The cooling airis at least 5° C. cooler than the film.

Certain terms are used in the description and the claims that, while forthe most part are well known, may require some explanation. “Biaxiallystretched,” when used herein to describe a film, indicates that the filmhas been stretched in two different directions, a first direction and asecond direction, in the plane of the film. Typically, but not always,the two directions are substantially perpendicular and are in themachine direction (“MD”) of the film and the transverse direction (“TD”)of the film. Biaxially stretched films may be sequentially stretched,simultaneously stretched, or stretched by some combination ofsimultaneous and sequential stretching. “Simultaneously biaxiallystretched,” when used herein to describe a film, indicates thatsignificant portions of the stretching in each of the two directions areperformed simultaneously. Unless context requires otherwise, the terms“orient,” “draw,” and “stretch” are used interchangeably throughout, asare the terms “oriented,” “drawn,” and “stretched,” and the terms“orienting,” “drawing,” and “stretching.”

The term “stretch ratio,” as used herein to describe a method ofstretching or a stretched film, indicates the ratio of a lineardimension of a given portion of a stretched film to the linear dimensionof the same portion prior to stretching. For example, in a stretchedfilm having an MD stretch ratio of 5:1, a given portion of unstretchedfilm having a 1 cm linear measurement in the machine direction wouldhave 5 cm measurement in the machine direction after stretching. In astretched film having a TD stretch ratio of 5:1, a given portion ofunstretched film having a 1 cm linear measurement in the transversedirection would have 5 cm measurement in the transverse direction afterstretching.

“Area stretch ratio,” as used herein, indicates the ratio of the area ofa given portion of a stretched film to the area of the same portionprior to stretching. For example, in a biaxially stretched film havingan area stretch ratio of 50:1, a given 1 cm² portion of unstretched filmwould have an area of 50 cm² after stretching.

The mechanical stretch ratio, also known as nominal stretch ratio, isdetermined by the unstretched and stretched dimensions, and cantypically be measured at the film grippers at the edges of the film usedto stretch the film in the particular apparatus being used. Globalstretch ratio refers to the overall stretch ratio of the film after theportions that lie near the grippers, and thus are affected duringstretching by the presence of the grippers, have been removed fromconsideration. The global stretch ratio can be equivalent to themechanical stretch ratio when the input unstretched film has a constantthickness across its full width (from gripper to gripper, crossweb) andwhen the effects of proximity to the grippers upon stretching are small.More typically, however, the thickness of the input unstretched film isadjusted so as to be thicker or thinner near the grippers than at thecenter of the film. When this is the case, the global stretch ratio willdiffer from the mechanical or nominal stretch ratio. These global ormechanical ratios are both to be distinguished from a local stretchratio. The local stretch ratio is determined by measuring a particularportion of the film (for example a 1 cm portion) before and afterstretching. When stretching is not uniform over substantially the entireedge-trimmed film, then the local ratio can be different from the globalratio. When stretching is substantially uniform over substantially theentire film (excluding the area immediately near the edges andsurrounding the grippers along the edges), then the local ratioeverywhere will be substantially equal to the global ratio. Unless thecontext requires otherwise, the terms first direction stretch ratio,second direction stretch ratio, MD stretch ratio, TD stretch ratio, andarea stretch ratio are used herein to describe the global stretch ratio.

The term “stretch profile” is meant to refer collectively to the valuesof all the variables of stretching the film, including overallthroughput rate of the tenter, and the stretch ratios and temperaturesas a function of position in the process, and to the techniques used toattain these values, such as air impingement velocities, clipaccelerations and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be further explained with reference to theappended Figures, wherein:

FIG. 1 is a top schematic view of a tenter apparatus for use with thepresent invention.

FIG. 2 is a plot of the caliper variation as a function of MD positionfor a center-sample and an edge-sample of the film of Example 11.

FIG. 3 is a plot of the caliper variation as a function of MD positionfor the center-samples of the film of Example 11 and the film of Example10.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a top schematic view of a tenter apparatus forcarrying out the methods of the present invention. The tenter ispreferably of the type disclosed in U.S. Pat. No. 5,051,225, “Method ofDrawing Plastic Film in a Tenter Frame,” Hommes et al., the entirecontents of which are incorporated herein. Tenter apparatus 10 includesa first side rail 12 and a second side rail 14 on which the driven clips22 and idler clips 24 ride. In FIG. 1, the driven clips 22 areillustrated schematically as boxes marked “X” while the idler clips 24are illustrated schematically as open boxes. Between pairs of drivenclips 22 on a given rail, there are one or more idler clips 24. Asillustrated, there are two idler clips 24 between each pair of clips 22on a given rail. One set of clips 22, 24 travels in a closed loop aboutfirst rail 12 in the direction indicated by the arrows at the ends ofthe rail. Similarly, another set of clips 22, 24 travels in a closedloop about second rail 14 in the direction indicated by the arrows atthe ends of the rail. The clips 22, 24 hold the film edges and propelfilm 26 in the direction shown by the arrow at the center of the film.At the ends of the rails 12, 14, the clips 22, 24 release the film 26.The clips then return along the outside of the rails to the entrance ofthe tenter to grip the cast web to propel it through the tenter. (Forclarity of illustration, the clips returning to the entrance on theoutside of the rails have been omitted from FIG. 1.) The stretched film26 exiting the tenter may be wound up for later processing or use, ormay be further processed before winding.

The polymer can be cast into sheet form as is known in the art, toprepare a web suitable for stretching to arrive at the preferred filmdescribed herein. The web can be a homopolymer, copolymer, blend,monolayer, or multilayer, as is known in the art. When makingpolypropylene films, a suitable method for casting a web is to feed theresin into the feed hopper of a single screw, twin screw, cascade, orother extruder system having extruder barrel temperatures adjusted toproduce a stable homogeneous melt. The polypropylene melt can beextruded through a sheet die onto a rotating cooled metal casting wheel.Optionally, the casting wheel can be partially immersed in afluid-filled cooling bath, or, also optionally, the cast web can bepassed through a fluid-filled cooling bath after removal from thecasting wheel. The web is then biaxially stretched according to thepreferred methods described herein. The extruded web is typicallyquenched, optionally re-heated by passing through an infrared heater,and fed to the clips 22, 24 on the first and second rails 12, 14, to bepropelled through the tenter apparatus 10. The optional infrared heatingand the gripping by the clips 22, 24 may occur in any order orsimultaneously.

The rails 12, 14 pass through three sections: preheat section 16;stretch section 18; and post-stretch treatment section 20. In thepreheat section 16, the film is heated to within an appropriatetemperature range to allow a significant amount of stretching withoutbreaking. The three functional sections 16, 18, 20 may be broken downfurther into zones. For example, in one preferred embodiment of atenter, the preheat section 16 includes zones Z1, Z2, and Z3, thestretch section 18 includes zones Z4, Z5, and Z6, and the post-stretchtreatment section 20 includes zones Z7, Z8, and Z9. It is understoodthat the preheat, stretch, and post-treatment sections may each includefewer or more zones than illustrated. Further, within the stretchsection 18, the TD component of stretch or the MD component of stretchmay be performed in the same or in different zones. For example, MD andTD stretch each may occur in any one, two or three of the zones Z4, Z5,and Z6. Further, one component of stretch may occur before the other, ormay begin before the other and overlap the other. Still further, eithercomponent of stretch may occur in more than one discrete step. Forexample, MD stretch may occur in Z4 and Z6 without any MD stretchoccurring in Z5.

Some stretching in the MD and/or TD may also occur in the preheatsection or post-stretch treatment section. For example, in theembodiment illustrated, stretching may begin in Zone 3. Stretching maycontinue into Zone 7 or beyond. Stretching may resume in any of theZones after Zones Z4, Z5, or Z6.

In one preferred stretch profile, the film is stretched to an MD stretchratio of at least 2:1 and a TD stretch ratio of at least 2:1. The finalstretch ratios may be selected to provide films having desiredcharacteristics and properties.

In one preferred stretch profile, simultaneous biaxial stretching occursin the stretch section 18. For example, TD stretch occurs throughoutzones Z4, Z5 and Z6. For this to occur, the first and second rails 12,14 are configured to diverge through each of these zones. In thisstretch profile, MD stretch preferably occurs only in zone Z4. For thisto occur, the driven clips 22 are accelerated through zone Z4 so as toinduce MD stretch, and then the spacing of the driven clips 22 ismaintained substantially constant in the MD through zones Z5 and Z6. Inanother preferred stretch profile, MD stretch occurs in zones Z4 and Z5,while TD stretch occurs in zones Z4, Z5, Z6. In yet another preferredstretch profile, both MD and TD stretch occur in zones Z4, Z5, and Z6.

In another preferred stretch profile, sequential biaxial stretchingoccurs. For MD stretch to precede TD, the rails 12, 14 can remainparallel in zone Z4 while the driven clips 22 accelerate in the MD. Therails 12, 14 then diverge in either or both of zones Z5 and Z6 for TDstretch while the MD spacing of the driven clips 22 remainssubstantially constant in these zones. For TD to precede MD, the rails12, 14 diverge initially with no or little MD stretch, and then remainparallel while MD stretch occurs.

Usually the film 26 is then propelled through the post-stretch treatmentsection 20. In this section, the film 26 typically is maintained at adesired temperature while no significant stretching occurs. Thistreatment is often referred to as a heat set or anneal, and is performedto improve the properties of the final film, such as the dimensionalstability. Alternatively, a small amount of relaxation in either or bothof the MD and TD may occur in the post-stretch treatment section 20.Relaxation here refers to a convergence of the rails in the TD and/or aconvergence of the driven clips on each rail in the MD.

Biaxial stretching of films is sensitive to many process conditions,including but not limited to the composition of the resin, film castingand quenching parameters, the time-temperature history while preheatingthe film prior to stretching, the stretching temperature used, thestretch profile used, and the rates of stretching. With the benefits ofthe teachings herein, one of skill in the art may adjust any or all ofthe parameters and thereby obtain films having desired properties andcharacteristics.

Some preferred stretching conditions are as follows for polypropylenefilm. Cast web thickness is preferably from about 0.2 to 12 mm, morepreferably from about 1 to 3 mm. The temperature of the IR heat sourceis high enough to impart the desired pre-heating to the cast web. Theair temperature in the preheat section 16 is preferably about 170 to220° C. The air temperature in the stretch section 18 and post-stretchtreatment section 20 is preferably about 150 to 170° C. The film itselfin stretch section 18 is preferably approximately 120 to 165° C. toallow significant stretching to occur without breaking, more preferablyapproximately 150 to 165° C. For polypropylene, final area stretch ratiois at least 16:1; more preferably from about 25:1 to 100:1. The MDstretch ratio and TD stretch ratio are selected as desired, and may ormay not be equal to each other.

The cooling of the present invention, whether edge cooling or zonecooling, may begin before or after the onset of stretching in thestretch section 18. If cooling begins before the onset of stretching, itshould continue after the onset of stretching into the stretch section18. As used herein, including the claims, the phrase, “imparting amachine direction cooling gradient to at least a portion of the width ofthe stretched film” means imparting a temperature gradient such that thefilm is cooler at the forward side of the cooled film portion and warmerat the rearward side of the cooled film portion. “Forward” means thedirection of film travel in the machine direction and “rearward” isopposite to the direction of film travel in the machine direction. Bystating that the gradient is applied to at least a portion of the“stretched film,” this means the gradient is present after stretchingbegins. The gradient may in addition be present prior to the onset ofstretching provided the gradient continues to be imparted, or isre-imposed, after stretching begins. The gradient may be imparted to thestretched film at any location of the stretch section and/or just afterthe stretch section. Preferably, the cooling, and therefore thegradient, begins at, or continues at least until, the end of the stretchsection 18 or the beginning of the post-stretch treatment section 20. Inone preferred embodiment, the cooling occurs at the latter portion ofthe stretch section 18 and in the beginning of the post-stretch section20. This would be, for example, in zones Z6 and Z7 for the embodimentillustrated in FIG. 1. In another preferred embodiment, cooling occursat the latter portion of the stretch section 18. For example, coolingcan occur in either or both of zones Z5 and Z6 in the apparatusillustrated in FIG. 1, or in the second half of zone Z4 and throughoutzones Z5 and Z6. In another preferred embodiment, cooling occursthroughout the stretch section 18, for example in zones Z4, Z5, and Z6of the tenter of FIG. 1. In another preferred embodiment, cooling canoccur at the beginning of the post-stretch section 20, such as in eitheror both of zones Z7 and Z8. If the MD stretching and TD stretching zonesdo not coincide with one another, then in one preferred embodiment,cooling occurs in both the MD and the TD stretching zones. In anotherpreferred embodiment, cooling occurs at the MD stretching zones only.

Cooling is provided to at least a portion of the width of the film 26.Preferably, cooling is provided by actively cooling either: i) the edgeportions 28 of the film in a zone or zones; or ii) the full width,including the edge portions 28 and the center portion 30, of the film ina zone or zones. In one preferred stretch profile, at the onset ofstretching, the edge portions of the film are maintained no hotter thanthe center portion of the film. This may be continued throughout thestretching process.

Preferably, cooling is provided by forced air convection. The coolingair must be cooler than the temperature of the film at the location theair is provided. Preferably, the cooling air is provided at atemperature and flow rate effective to cool the film by at least 3° C.,more preferably 5° C., and still more preferably 10° C. The differenceof the temperature of the cooling air and that of the film to be cooledis called the air temperature differential and should be at least 5° C.,and may be significantly greater. The difference of the temperature ofthe film with and without cooling is called the target film temperaturedifferential. Usually, due to the nature of heat transfer, the edge airor zone air temperature differential is greater than the target filmtemperature differential. The cooling imparts a temperature drop in thefilm in the machine direction such that, when viewed from a locationupon the film, the film is cooler in the direction of film travel thanin the direction opposite film travel. The preferred temperature of thecooling air will depend on factors such as film temperature, thickness,speed, and heat transfer characteristics of the tenter. The temperatureand location of the cooling air can be selected by one of skill in theart with reference to the teachings of the present invention to obtainthe desired improvements disclosed herein.

The cooling is provided at a location and temperature effective toimprove uniformity of the spacing of the idler clips and driven clipscompared to the spacing obtained at otherwise identical conditionswithout such cooling. Spacing uniformity is determined as follows. Thespacing between the clips can be determined, for example, bymeasurements on the stretched film 26. The ideal clip spacing isdefined, for a system with two idler clips between each pair of drivenclips on each rail, as one-third of the spacing between successivedriven clips D₁ (forward—toward the tenter exit) and D₂ (rearward—towardthe tenter entrance). If there are N idler clips between driven clips D₁and D₂, each nearest-neighbor pair of clips, D₁-I₁, I₁-I₂, . . . throughI_(N)-D_(2,) should have an ideal spacing of 1/(N+1) of the distanceD1-D2. A numerical value for the non-uniformity of the spacing can beobtained by measuring the actual pairwise spacings obtained, subtractingfrom the measured spacing of each nearest-neighbor pair the idealspacing, taking the absolute value of each difference, and summing.Ideal spacing, therefore, will give a value for spacing non-uniformityof zero. Larger values represent increasing spacing non-uniformity. Animprovement in spacing uniformity will manifest as a decrease in thevalue of the spacing non-uniformity. Preferably, spacing non-uniformityis decreased by at least 5% of what it would have been without thecooling. More preferably, non-uniformity is decreased by at least 10%,and still more preferably by at least 50%. Alternatively, cooling isprovided at a location and temperature effective to provide that theclip spacing of each nearest-neighbor pair is within 20% of ideal, morepreferably within 10% of ideal, and most preferably within 5% of ideal.In one preferred embodiment using polypropylene, when the tentertemperature is set to approximately 160 to 165° C., cooling air for edgecooling is approximately 30 to 140° C., more preferably about 65 to 120°C., and still more preferably about 70 to 110° C. In one preferred zonecooling embodiment using polypropylene, when the tenter temperature isset to approximately 160 to 165° C., cooling air is approximately 100 to150° C., more preferably about 120 to 140° C., and still more preferablyabout 125 to 130° C. With the benefits of the teachings herein, one ofskill in the art can select edge cooling and zone cooling parameters forother materials, thicknesses, film speeds, tenter temperatures, andother stretch profiles.

In another preferred stretch profile, cooling is provided to at least aportion of the width of the film in an effective amount to reduce thevalue of idler clip lag from the value of idler clip lag obtained atotherwise identical conditions in the absence of said cooling. Clip lagvalues are determined as follows. The spacing between the clips can bedetermined, for example, by measurements on the stretched film 26. Theideal clip spacing is defined, for a system with two idler clips betweeneach pair of driven clips on each rail, as one-third of the spacingbetween successive driven clips D₁ (forward—toward the tenter exit) andD₂ (rearward—toward the tenter entrance). Idler clip I₁ is the forwardof the two idler clips between driven clips, and idler clip I₂ is therearward of the two. The values for pairs D₁-I₁, I₁-I₂, and I₂-D₂, aspercent variations in spacing from ideal (with respect to the ideal) arecalculated, with positive numbers indicating spacings farther thanideal, and negative numbers indicating spacings closer than ideal. D₁-I₁indicates the percent spacing variation from ideal between the forwarddriven and forward idler clips, I₁-I₂ is the percent spacing variationfrom ideal between idler clips, and I₂-D₂ the spacing variation fromideal between the rear idler clip and the rear driven clip. The totalclip lag value reported is calculated as the percent variation fromideal spacing for D₁-I₁, minus the percent variation from ideal forI₂-D₂. This calculation can be extended to cases with differing numbersof idler clips between each pair of driven clips. For the case of onlyone idler clip between each pair of driven clips, I₁ equals I₂, and thecalculation outlined above may proceed on that basis. For the case ofN>2 idler clips, I₂ in the formulation above becomes I_(N), and thecalculation may proceed on that basis. Spacings between any two idlerclips are ignored in the calculation of idler clip lag regardless of thenumber of idlers present.

Preferably, idler clip lag is decreased by at least 5% of what it wouldhave been at otherwise identical conditions without the cooling. Morepreferably, idler clip lag is decreased by at least 10%, and still morepreferably by at least 50%. Alternatively, cooling is provided at alocation and temperature effective to provide that the value of idlerclip lag is less than about 20%, more preferably less than about 10%,and most preferably less than about 5%.

A negative value for clip lag, thus defined, is indicative of clip lead.Preferably, clip lag approaches zero. In some cases, it may bepreferable to impart clip lead. As used herein, including the claims,the phrase, “reduce the value of idler clip lag” is meant to indicatethat the value will be made either a smaller positive number, zero, orany negative number (clip lead). To denote specifically an approachtoward the ideal (uniform) clip separation, the phrase “reduce theabsolute value of idler clip lag” will be used.

In another preferred stretch profile, cooling is provided to at least aportion of the width of the film in an effective amount to improve thecaliper uniformity relative to the caliper uniformity obtained atotherwise identical conditions in the absence of the cooling. Caliperuniformity may be measured either across the web, e.g. from clip face toclip face, or down the web, e.g. along the direction of film travel.Either or both of the crossweb and downweb caliper uniformity may beimproved. The non-uniformity may be characterized by the standarddeviation from the mean of a caliper scan along a given direction.Alternatively, the maximum peak to valley height of a caliper scan alonga given direction may be used. A perfectly uniform film would have anon-uniformity of zero. A variety of caliper measuring techniques may beused. Typically, the higher the resolution, the better. A preferredmeasurement technique is to cut crossweb or downweb strips and then scanthe caliper using a PC 5000 Electronic Thickness Gauge available fromElectro-Gauge Inc., located in Eden Prairie, Minn., USA. Crosswebuniformity may also be characterized by comparing a series ofdownweb-cut strips cut along “lanes” differing in crossweb position.

FIG. 2 presents such a pair of caliper scans. The marks on the MDPosition axis represent the positions of the driven clips relative tothe film samples. The data of FIG. 2 is taken from a film made in aprocess with two idler clips between each pair of driven clips accordingto Example 11 below. The edge lane (plot E₁₁) was located about 16% ofthe way across the film from a clip face whereas the center lane (plotC₁₁) was 50% of the way across the film (centered). Total clip lag wasmeasured as 58%. FIG. 2 shows that there is a relationship betweencaliper non-uniformity and clip lag. The caliper non-uniformity isperiodic with a “wavelength” roughly equal to the final separation ofthe driven clips. FIG. 2 also shows that the magnitude of calipernon-uniformity decreases from the edge of the film near the clipstowards the center of the film. A downweb strip cut along a lane nearthe edge has higher non-uniformity than a downweb strip cut along a lanenear the center, though the periodic nature of the caliper fluctuationremains. Increasing the initial web width may increase the width of acentral portion with relatively low non-uniformity; nevertheless, cliplagging will occur in films having lower yield (the fraction of thewidth which is usable width).

FIG. 3 shows that the non-uniformity decreases for center lanes withdecreasing clip lagging. Therefore, reduced clip lagging, or variationfrom the ideal clip spacing, is observed in more uniform films and/or infilms in which a larger portion of the width has good uniformity,thereby increasing the yield for a given caliper uniformityspecification. Caliper traces shown represent 58% lagging (plot C₁₁, ofExample 11 below) and less than 2% lagging (plot C₁₀, of Example 10below). Plot C₁₀ of Example 10 does not show the same periodicity basedon driven-clip separations. Clip position does not correlate stronglywith caliper non-uniformity in this example with low values of clip lagand caliper non-uniformity.

It will be readily appreciated that idler clip lagging, or anynon-uniformity of clip spacing, occurs when there is downweb calipernon-uniformity. Typical polymeric films drawn above the glass transitiontemperature are nearly volume preserving, except through voiding or viadensification due to crystallization, so that the decrease in thicknessis approximately proportional to the product of the local principal drawratios, e.g. the local crossweb and downweb draw ratios. The presentinvention also recognizes the link between caliper and draw rationonuniformities and non-uniformity of other properties both crossweb anddownweb. These physical, mechanical and optical properties include butare not limited to elastic moduli, tensile strength, elongation atbreak, energy-to-break per unit volume and other tear and dispensingproperties, surface characteristics, interlayer adhesion in multilayerfilms, coefficients of thermal and hygroscopic expansion, heatshrinkage, refractive indices, capacitance and other dielectricproperties, haze, transparency, color, spectral band edges, and otheroptical measures of appearance and performance. By dispensing propertiesis meant the properties relating to the ease of severing and the qualityof severed edge when a film, converted into the form of a tape, isdispensed using a dispenser having a cutting edge. The level ofnon-uniformity of these various properties may be related to the caliperfluctuations and clip lag, for example, through differing sensitivitiesof these properties to the local caliper and local draw ratios. Thus,lagging is symptomatic of a downweb draw ratio fluctuation which causesboth a downweb caliper fluctuation and a downweb modulus of elasticityfluctuation. Caliper may fluctuate differently than modulus because ofcorresponding partial compensation of the thickness by concomitantcrossweb draw ratio fluctuations under certain conditions as well as thenonlinear relationship between moduli and draw ratios.

Although the present invention is described herein with particularapplicability to methods of biaxially stretching films and to resultingbiaxially stretched films, the present invention may also be appliedadvantageously to methods of stretching films in a single directionunder conditions in which the film is held by clips, and the clips areseparated along the machine direction, and thus capable of producingidler clip lagging or leading. In one such method, the film is stretchedsolely along the machine direction thus separating the clips along themachine direction and creating the possibility of clip lagging. Inanother example, the clips begin the draw with some MD separation, andthen stretching in the transverse direction, for example, may createnon-uniformity in the MD clip spacing.

The methods of stretching with appropriate cooling described herein arewell suited for use on films including a polymeric film. Preferably, thefilm comprises a thermoplastic polymer. For a film having more than onelayer, the description of suitable materials which follows need applyonly to one of the layers. Suitable polymeric film materials for use inthe current invention include thermoplastics capable of being formedinto biaxially oriented films. Suitable thermoplastic polymer filmmaterials include, but are not limited to, polyesters, polycarbonates,polyarylates, polyamides, polyimides, polyamide-imides,polyether-amides, polyetherimides, polyaryl ethers, polyaryletherketones, aliphatic polyketones, polyphenylene sulfide, polysulfones,polystyrenes and their derivatives, polyacrylates, polymethacrylates,cellulose derivatives, polyethylenes, aliphatic and cycloaliphaticpolyolefins, copolymers having a predominant olefin monomer, fluorinatedpolymers and copolymers, chlorinated polymers, polyacrylonitrile,polyvinylacetate, polyvinylalcohol, polyethers, ionomeric resins,elastomers, silicone resins, epoxy resins, and polyurethanes. Miscibleor immiscible polymer blends including any of the above-named polymers,and copolymers having any of the constituent monomers of any of theabove-named polymers, are also suitable, provided a biaxially orientedfilm may be produced from such a blend or copolymer.

Preferred among thermoplastics are the vinyl polymers, by which is meantall polymers of the general formula —[CWX—CYZ]_(n)—, where W, X, Y, andZ are either hydrogen (H) or any substituent atoms or groups. Thuswithin the preferred vinyl polymer class we include thetetrasubstituted, trisubstituted, 1,2-disubstituted and1,1-disubstituted polymers (including the “vinylidene” polymers) as wellas the more common monosubstituted vinyl polymers. Examples include thepolyolefins, polyvinyl chloride, polyvinyl fluoride, polyvinylidenechloride, polyvinylidene fluoride, polytrifluoroethylene,polychlorotrifluoroethylene, polyvinyl acetate, polyvinyl alcohol,polyacrylic acid and its esters, polyacrylonitrile, and polymethacrylicacid and its esters (such as polymethyl methacrylate).

More preferred are the polyolefins, by which is meant all polymers ofthe general formula —[CH₂CR¹R²]_(n)—, where R¹ and R² are saturated orunsaturated, linear or branched alkyl, cycloalkyl, or aryl groups, orhydrogen. Included are such polymers as polyethylenes, polypropylenes,polybutene-1, poly-(4-methylpentene-1), polyisobutene,poly-(vinylcyclohexane), polybutadienes, and polystyrene and its ring-and alpha-substituted derivatives.

Still more preferred are polyethylene and the saturated alkyl orcycloalkyl polyolefins. Polypropylene is most preferred.

The methods of stretching with appropriate cooling described herein arewell suited for use on films including amorphous or semi-crystallinethermoplastic polymeric films. Amorphous thermoplastics include, but arenot limited to, polymethacrylates, polycarbonates, atactic polyolefinsand random copolymers. Semi-crystalline thermoplastics include, but arenot limited to, polyesters, polyamides, thermoplastic polyimides,polyarylether ketones, aliphatic polyketones, polyphenylene sulfide,isotactic or syndiotactic polystyrene and their derivatives,polyacrylates, polymethacrylates, cellulose derivatives, polyethylene,polyolefins, fluorinated polymers and copolymers, polyvinylidenechloride, polyacrylonitrile, polyvinylacetate, and polyethers.

Semicrystalline thermoplastics from which biaxially oriented films maybe produced are sometimes characterized in terms of their degree ofcrystallinity at various stages in the film-making process. Thus,polyethylene terephthalate (PET), a common polymer for biaxiallyoriented film, is well-known to be quenchable when cast into a film.That is, PET crystallizes slowly enough that it can be extruded onto achilled roll and thereby cooled below its glass transition temperaturesufficiently quickly to prevent the formation of measurable amounts ofcrystallinity. It is well known that such quenching is advantageous forthe production of biaxially oriented PET film, both because it enablesthe stretching step(s) to take place at temperatures only slightly abovethe glass transition, and because it allows a significant amount ofstretching without breaking, which breaking is prevalent if a morebrittle semicrystalline cast web is allowed to form.

The degree of crystallinity of a semicrystalline polymer film isdifficult to precisely quantify, as it depends not only on theassumption of a two-phase model (crystalline and amorphous) for polymermorphology which may or may not be precisely accurate, but also on theassumption of the constancy of some measurable property (density, forexample) for each phase regardless of such variables as degree oforientation. Different measurement techniques frequently providedifferent results due to the inadequacies of these assumptions. Thus,precise agreement among workers is not to be expected, especially wheredifferent techniques have been employed. Techniques well-known in theart for estimating the degree of crystallinity include density,differential scanning calorimetry (DSC), average refractive index(through its relationship to the density), analysis of infrared bands,and X-ray methods.

Usually, the degree of crystallinity of PET in the form of unstretchedcast film is reported to be undetectably low, or 0%, or below 1%. Thisis typically referred to as an amorphous cast web. In a simultaneousbiaxial orientation process, film of this low degree of crystallinitywould be fed to the tenter. In the more commonly employed sequentialprocess, however, such an amorphous film is first stretched in themachine direction using heated rolls rotating at different speeds. Such“length orientation” imparts some crystallinity to the film, the degreeof which has been reported at anywhere from 7% to 30%. See LeBourvellecand Beautemps, J. Appl. Polym. Sci. 39, 329-39 (1990); and Faisant deChampchesnel, et al., Polymer 35(19), 4092-4102 (1994). Typical valuesin commercial practice range from 10-20%. See Encycl. Of Polym. Sci. &Engrg., vol. 12, Wiley (NY) 1988, pg. 197. In a sequential process, itis film of this degree of crystallinity which would be fed to thetenter. Transverse direction stretching in the tenter has been reportedto increase the degree of crystallinity to within the range of 17% to40%. Subsequent heat-setting or annealing under transverse constraint inthe tenter is reported to further increase the degree of crystallinityto about 45% to 50%. The breadth of the range reported for pre-heatsetfilm is due both to the range of crystallinities of the length-orientedfilms provided as input to that step of the process, and to theexperimental difficulty of decoupling the transverse directionstretching step from the heat-setting step, both of which occur withinthe tenter oven. Considerably less is known regarding the behavior ofPET in a simultaneous biaxial orientation process, but the availabledata places the degrees of crystallinity after stretching and afterheat-setting in the same ranges as those for the sequential processafter TD stretching and after heat-setting.

Another polyester suitable for use with the present invention ispolyethylene naphthalate (PEN). PEN is known to crystallize somewhatmore slowly than PET. Nonetheless, reports of its behavior intenter-film processes place the degrees of crystallinity at the end ofeach process step in roughly the same ranges as those reported for PET.Thus, when processed conventionally, PEN too is an example of anamorphous cast web.

In contrast to the polyesters, polypropylene (PP) crystallizes sorapidly that it is almost impossible to quench the molten polymer toless than 50% crystallinity with any practical commercial method. SeeThe Science and Technology of Polymer Films, Vol. II, by Orville J.Sweeting, Wiley (NY), 1971, pg. 223. As a result, PP is stretched attemperatures just below the crystalline melting point, rather than attemperature just above the glass transition as is the case for thepolyesters. Some additional crystallinity develops during the process,but the amount is small. One comprehensive study found the degrees ofcrystallinity of PP cast (unstretched) film, length-oriented film, andsequentially biaxially oriented film to be 58%, 62% and 70%,respectively. See A. J. deVries, Pure Appl. Chem. 53, 1011-1037 (1981).The Encyclopedia of Polym. Sci. & Engrg., Vol. 7, Wiley (NY), 1987, pg.80, reports the degree of crystallinity of typical biaxially oriented PPfilms at 65-70%.

The methods of stretching with appropriate cooling described herein arewell suited for use on films including semicrystalline thermoplasticpolymer films. Preferred semicrystalline thermoplastic polymers arethose which can undergo a significant amount of stretching withoutbreaking when the film entering the tenter inlet has a degree ofcrystallinity greater than about 1%. Such films are referred to hereinas pre-crystallized polymeric films. More preferred semicrystallinethermoplastic polymers are those which can be effectively biaxiallystretched without breaking when the film entering the tenter inlet has adegree of crystallinity greater than about 7%. Still more preferredsemicrystalline thermoplastic polymers are those which can beeffectively biaxially stretched without breaking when the film enteringthe tenter inlet has a degree of crystallinity greater than about 30%.Even more preferred semicrystalline thermoplastic polymers are thosewhich can be effectively biaxially stretched without breaking when thefilm entering the tenter inlet has a degree of crystallinity greaterthan about 50%. Polypropylene is most preferred.

For the purposes of the present invention, the term “polypropylene” ismeant to include copolymers having at least about 90% propylene monomerunits, by weight. “Polypropylene” is also meant to include polymermixtures having at least about 65% polypropylene, by weight.Polypropylene for use in the present invention is preferablypredominantly isotactic. Isotactic polypropylene has a chainisotacticity index of at least about 80%, an n-heptane soluble contentof less than about 15% by weight, and a density between about 0.86 and0.92 grams/cm³ measured according to ASTM D1505-96 (“Density of Plasticsby the Density-Gradient Technique”). Typical polypropylenes for use inthe present invention have a melt flow index between about 0.1 and 15grams/ten minutes according to ASTM D1238-95 (“Flow Rates ofThermoplastics by Extrusion Plastometer”) at a temperature of 230° C.and force of 21.6 N, a weight-average molecular weight between about100,000 and 400,000, and a polydispersity index between about 2 and 15.Typical polypropylenes for use in the present invention have a meltingpoint as determined using differential scanning calorimetry of greaterthan about 130° C., preferably greater than about 140° C., and mostpreferably greater than about 150° C. Further, the polypropylenes usefulin this invention may be copolymers, terpolymers, quaterpolymers, etc.,having ethylene monomer units and/or alpha-olefin monomer units havingbetween 4-8 carbon atoms, said comonomer(s) content being less than 10%by weight. Other suitable comonomers include, but are not limited to,1-decene, 1-dodecene, vinylcyclohexene, styrene, allylbenzene,cyclopentene, norbornene, and 5-methylnorbornene. One suitablepolypropylene resin is an isotactic polypropylene homopolymer resinhaving a melt flow index of 2.5 g/10 minutes, commercially availableunder the product designation 3374 from FINA Oil and Chemical Co.,Dallas, Tex. Recycled or reprocessed polypropylene in the form, forexample, of scrap film or edge trimmings, may also be incorporated intothe polypropylene in amounts less than about 60% by weight.

As already mentioned, mixtures having at least about 65% isotacticpolypropylene and at most about 35% of another polymer or polymers mayalso be advantageously used in the process of the present invention.Suitable additional polymers in such mixtures include, but are notlimited to, propylene copolymers, polyethylenes, polyolefins havingmonomers having from four to eight carbon atoms, and other polypropyleneresins.

Polypropylene for use in the present invention may optionally include1-40% by weight of a resin, of synthetic or natural origin, having amolecular weight between about 300 and 8000, and having a softeningpoint between about 60° C. and 180° C. Typically, such a resin is chosenfrom one of four main classes: petroleum resins, styrene resins,cyclopentadiene resins, and terpene resins. Optionally, resins from anyof these classes may be partially or fully hydrogenated. Petroleumresins typically have, as monomeric constituents, styrene,methylstyrene, vinyltoluene, indene, methylindene, butadiene, isoprene,piperylene, and/or pentylene. Styrene resins typically have, asmonomeric constituents, styrene, methylstyrene, vinyltoluene, and/orbutadiene. Cyclopentadiene resins typically have, as monomericconstituents, cyclopentadiene and optionally other monomers. Terpeneresins typically have, as monomeric constituents, pinene, alpha-pinene,dipentene, limonene, myrcene, and camphene.

Polypropylene for use in the present invention may optionally includeadditives and other components as is known in the art. For example, thefilms of the present invention may contain fillers, pigments and othercolorants, antiblocking agents, lubricants, plasticizers, processingaids, antistatic agents, nucleating agents, antioxidants and heatstabilizing agents, ultraviolet-light stabilizing agents, and otherproperty modifiers. Fillers and other additives are preferably added inan effective amount selected so as not to adversely affect theproperties attained by the preferred embodiments described herein.Typically such materials are added to a polymer before it is made intoan oriented film (e.g., in the polymer melt before extrusion into afilm). Organic fillers may include organic dyes and resins, as well asorganic fibers such as nylon and polyimide fibers, and inclusions ofother, optionally crosslinked, polymers such as polyethylene,polyesters, polycarbonates, polystyrenes, polyamides, halogenatedpolymers, polymethyl methacrylate, and cycloolefin polymers. Inorganicfillers may include pigments, fumed silica and other forms of silicondioxide, silicates such as aluminum silicate or magnesium silicate,kaolin, talc, sodium aluminum silicate, potassium aluminum silicate,calcium carbonate, magnesium carbonate, diatomaceous earth, gypsum,aluminum sulfate, barium sulfate, calcium phosphate, aluminum oxide,titanium dioxide, magnesium oxide, iron oxides, carbon fibers, carbonblack, graphite, glass beads, glass bubbles, mineral fibers, clayparticles, metal particles and the like. In some applications it may beadvantageous for voids to form around the filler particles during thebiaxial orientation process of the present invention. Many of theorganic and inorganic fillers may also be used effectively asantiblocking agents. Alternatively, or in addition, lubricants such aspolydimethyl siloxane oils, metal soaps, waxes, higher aliphatic esters,and higher aliphatic acid amides (such as erucamide, oleamide,stearamide, and behenamide) may be employed.

Antistatic agents may also be employed, including aliphatic tertiaryamines, glycerol monostearates, alkali metal alkanesulfonates,ethoxylated or propoxylated polydiorganosiloxanes, polyethylene glycolesters, polyethylene glycol ethers, fatty acid esters, ethanol amides,mono- and diglycerides, and ethoxylated fatty amines. Organic orinorganic nucleating agents may also be incorporated, such asdibenzylsorbitol or its derivatives, quinacridone and its derivatives,metal salts of benzoic acid such as sodium benzoate, sodiumbis(4-tert-butyl-phenyl)phosphate, silica, talc, and bentonite.Antioxidants and heat stabilizers, including phenolic types (such aspentaerythrityl tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene),and alkali and alkaline earth metal stearates and carbonates may also beadvantageously used. Other additives such as flame retardants,ultraviolet-light stabilizers, compatibilizers, antimicrobial agents(e.g., zinc oxide), electrical conductors, and thermal conductors (e.g.,aluminum oxide, boron nitride, aluminum nitride, and nickel particles)may also be blended into the polymer used to form the film.

Resulting films have desirably uniform properties and are well suitedfor many applications. One preferred application for the film of thepresent application is as a tape backing. Preferably, the tape backinghas a thickness in the range of about 0.020 to about 0.064 mm. Thebacking is coated with a layer of any suitable adhesive as is known inthe art. The backing may have an optional release or low adhesionbacksize layer as is known in the art.

The adhesive may be any suitable adhesive as is known in the art.Preferred adhesives are those activatable by pressure, heat orcombinations thereof. Suitable adhesives include those based onacrylate, rubber resin, epoxies, urethanes or combinations thereof. Theadhesive may be applied by solution, water-based or hot-melt coatingmethods. The adhesive can include hot melt-coated formulations,transfer-coated formulations, solvent-coated formulations, and latexformulations, as well as laminating, thermally-activated, andwater-activated adhesives and bonding agents. Useful adhesives includepressure sensitive adhesives. Pressure sensitive adhesives are wellknown to possess properties including: aggressive and permanent tack,adherence with no more than finger pressure, and sufficient ability tohold onto an adherend. Examples of useful adhesives include those basedon general compositions of polyacrylate; polyvinyl ether; diene rubbersuch as natural rubber, polyisoprene, and polybutadiene;polyisobutylene; polychloroprene; butyl rubber; butadiene-acrylonitrilepolymer; thermoplastic elastomer; block copolymers such asstyrene-isoprene and styrene-isoprene-styrene (SIS) block copolymers,ethylene-propylene-diene polymers, and styrene-butadiene polymers;poly-alpha-olefin; amorphous polyolefin; silicone; ethylene-containingcopolymer such as ethylene vinyl acetate, ethylacrylate, and ethylmethacrylate; polyurethane; polyamide; epoxy; polyvinylpyrrolidone andvinylpyrrolidone copolymers; polyesters; and mixtures or blends(continuous or discontinuous phases) of the above. Additionally, theadhesives can contain additives such as tackifiers, plasticizers,fillers, antioxidants, stabilizers, pigments, diffusing materials,curatives, fibers, filaments, and solvents. Also, the adhesiveoptionally can be cured by any known method.

A general description of useful pressure sensitive adhesives may befound in Encyclopedia of Polymer Science and Engineering, Vol. 13,Wiley-Interscience Publishers (New York, 1988). Additional descriptionof useful pressure sensitive adhesives may be found in Encyclopedia ofPolymer Science and Technology, Vol. 1, Interscience Publishers (NewYork, 1964).

The film for tape backing may be optionally treated by exposure to flameor corona discharge or other surface treatments including chemicalpriming to improve adhesion of subsequent coating layers.

The operation of the present invention will be further described withregard to the following detailed examples. These examples are offered tofurther illustrate the various specific and preferred embodiments andtechniques. It should be understood, however, that many variations andmodifications may be made while remaining within the scope of thepresent invention.

EXAMPLES 1-6

The following examples were prepared on a linear motor tenter generallyas described in the '225 patent discussed above, which had two idlerclips between each pair of driven clips. A continuous polypropylene castsheet (Fina 3374x, from Fina Chemical, Houston, Tex.) was extruded at athickness of approximately 0.054 inches (1.4 mm) and a width of 9.6inches (244 mm), and quenched on a chill roll/water bath system. Thefilm was passed between a set of infrared heaters (IR heater), then intoa linear motor tenter oven. The IR heat temperature, oven preheatsection temperatures (Zones 1-3), and stretch section temperatures(Zones 4-6) are set forth in Table 1. Web temperature as measured by anIR pyrometer at the entrance to Zone 4 at the beginning of the stretchsection is also reported in Table 1. For Examples 1-6, the post-stretchtreatment temperatures were as follows: Zone 7: 160° C.; Zones 8 and 9:165° C. Also, in each of these examples, the final stretch ratios were7:1 in the MD and 7:1 in the TD. The Zones in which MD stretch occurred(4, 4-5, or 4-6) are reported below. In each of these examples, TDstretch was performed in Zones 4 through 6. All of these stretchprofiles were linear with respect to machine position, and include a 10%stretch relaxation in both directions that occurred in Zones 8 and 9.Examples 1-3 had edge cooling air turned off. Examples 4-6 included edgecooling air, and otherwise correspond to Examples 1-3, respectively.TABLE 1 Stretching Conditions Preheat Section Temp. (C.) Stretch SectionTemp. (C.) Web Temp. MD Stretch Edge Cool Air Temp. (C.) Ex. IR Heat(C.) Z1 Z2 Z3 Z4 Z5 Z6 (C.) Zones Z6 Z7 1 700 185 178 178 165 164 163144.5 4 — — 2 740 192 186 176 162 161 160 — 4-5 — — 3 680 193 185 180166 164 163 148 4-6 — — 4 700 185 178 178 165 164 163 144.1 4 73.2 75.35 740 192 186 176 162 161 160 — 4-5 140.5 85.1 6 680 193 185 180 166 164163 148 4-6 74.4 77.8

The spacing between the clips was measured on the output film and theidler clip lagging calculated and reported in Table 2. The ideal idlerclip spacing is defined as one-third of the spacing between successivedriven clips D₁ (forward) and D₂ (rearward). Idler clip I₁ is theforward of the two idler clips between driven clips, and idler clip I₂is the rearward of the two. The values for D₁-I₁, I₁-I₂, and I₂-D₂ inTable 2 are the percent variation in spacing from ideal, with positivenumbers indicating a spacing farther than ideal, and negative numbersindicating spacing closer than ideal. D₁-I₁ indicates the percentspacing variation between the forward driven and forward idler clips,I₁-I₂ is the percent spacing variation between idler clips, and I₂-D₂the spacing variation between the rear idler clip and the rear drivenclip. The Total Lag reported is the percent variation from ideal spacingof D₁ to I₁, minus the percent variation from ideal for I₂-D₂. Theeffects of rounding cause some of the values in the “Total” columns InTable 2 to deviate from the differences of the D₁-I₁ and I₂-D₂ columnsby one unit in the last decimal place. All values are reported for boththe set of clips on the First Side of the tenter and on the oppositeSecond Side of the tenter. TABLE 2 First Side Second Side Ex. D₁-I₁I₁-I₂ I₂-D₂ Total D₁-I₁ I₁-I₂ I₂-D₂ Total 1 3.5 −1.0 −2.5 6.1 2.1 −1.1−1.0 3.1 2 12.8 −1.7 −11.1 23.9 8.6 −1.5 −7.1 15.7 3 5.3 −0.3 −5.0 10.33.7 0.0 −3.7 7.5 4 2.7 −0.7 −2.1 4.8 0.4 −0.5 0.1 0.3 5 11.0 −1.4 −9.520.5 6.4 −1.0 −5.4 11.8 6 −1.7 −1.0 2.7 −4.4 −2.5 −0.9 3.4 −5.9

From the results presented in Table 2, it can be seen that idler cliplagging in Example 1 of 6.1 on one side and 3.1 on the other side can bereduced to 4.8 and 0.3 respectively, with the addition of edge coolingin Example 4. Furthermore, idler clip lagging in Example 2 of 23.9 and15.7, can be reduced to 20.5 and 11.8, respectively, with the additionof edge cooling in Example 5. Also, idler clip lagging of 10.3 and 7.5of Example 3 can be changed to idler clip lead of −4.4 and −5.9 with theaddition of edge cooling in Example 6. The Examples also suggest that,if idler clip lag can be reduced (examples 4 and 5), or changed to idlerclip lead (example 6), a set of edge cooling conditions can be foundwhich would lead to ideal idler clip spacing.

EXAMPLES 7-10

The following examples were prepared on a linear motor tenter generallyas described in the '225 patent discussed above, which had two idlerclips between each pair of driven clips. A continuous polypropylene castsheet (Fina 3374x, from Fina Chemical, Houston, Texas) was extruded at athickness of approximately 0.054 inches (1.36 mm) and 13.8 inches wide(350 mm), and quenched on a chill roll/water bath system. The film waspassed between a set of infrared heaters (IR heater), then into a linearmotor tenter oven. For examples 7-10, the IR heat temperature was set at500° C., oven preheat zone temperatures (Zones 1-3) were set at 207° C.,205° C., and 193 ° C. respectively, and the stretch zone temperatures(Zones 4-5) were set at 160° C. and 155° C. respectively. The relaxation(Zone 6) and the post-stretch treatment (Zones 7-9) temperatures wereset as listed in Table 3. In each of these examples, the final stretchratios were 6.3:1 in the MD and 6.3:1 in the TD. The MD and TD stretcheswere performed simultaneously in Zones 4 and 5. All of these stretchprofiles were linear with respect to machine position, and include a 10%stretch relaxation in both MD and TD that occurred in Zone 6.

EXAMPLE 7

Example 7 included cooling air in Zone 6 that was 5° C. cooler than thetemperature of Zone 5.

EXAMPLE 8

Example 8 was prepared according to Example 7, with the exception of theuse of 15° C. cooling in Zone 6.

EXAMPLE 9

Example 9 was prepared according to Example 7, with the exception of theuse of 20° C. cooling in Zone 6.

EXAMPLE 10

Example 10 was prepared according to Example 7, with the exception ofthe use of 25° C. cooling in Zone 6 and an additional 5° C. in zone 7.TABLE 3 Stretching Conditions Relax Zone Temp.(° C.) Anneal ZoneTemperature (° C.) Example Zone 6 Zone 7 Zone 8 Zone 9 7 150 150 140 1308 140 140 140 130 9 135 135 135 130 10 130 125 125 125

The spacing between the clips was measured on the output film and theidler clip lagging calculated and reported in Table 4 as discussedearlier. All values are reported for both the sets of clips on the FirstSide of the tenter and on the opposite Second Side of the tenter. TABLE4 First Side Second Side Ex. D₁-I₁ I₁-I₂ I₂-D₂ Total D₁-I₁ I₁-I₂ I₂-D₂Total 7 7.9 −1.4 −6.3 14.2 4.8 −1.1 −3.7 8.5 8 3.7 −1.6 −2.2 5.9 2.0−1.6 −.4 2.4 9 0.6 0.6 −1.2 1.8 2.6 0.2 −1.6 4.1 10 1.3 −1.7 0.4 0.9−0.7 −1.3 1.9 −2.6

From the results presented in Table 4, it can be seen that idler cliplagging in Example 7 of 14.2 on one side and 8.5 on the other side canbe decreased with the addition of a sufficient amount of zone cooling,after the onset of stretching, as shown in Examples 8-10. In particular,as shown with Examples 9 and 10, the amount of overall lagging is lessthan 5%.

EXAMPLE 11

Example 11 was prepared on a linear motor tenter generally as describedin the '225 patent discussed above, which had two idler clips betweeneach pair of driven clips. A continuous polypropylene cast sheet (Fina3374x, from Fina Chemical, Houston, Tex.) was extruded at a thickness ofapproximately 0.055 inches (1.39 mm) and 13.8 inches wide (350 mm), andquenched on a chill roll/water bath system. The film was passed betweena set of infrared heaters (IR heater), then into a linear motor tenteroven. The IR heat temperature was set at 600° C., oven preheat zonetemperatures (Zones 1-3) were set at 184° C., 177° C., and 156° C.respectively, and the stretch zone temperatures (Zones 4-5-6-7 were setat 152° C., 170° C., 170° C., and 170° C. respectively. The relaxation(Zone 8) and the post-stretch treatment (Zone 9) temperatures were bothset at 158C. In this example, the final stretch ratios were 5.8:1 in theMD and 9.0:1 in the TD. The MD stretch was performed in Zones 4 and 5and the TD stretch was performed in Zones 4 through 7. The stretchprofile includes a 10% stretch relaxation in both directions in Zone 8.

The tests and test results described above are intended solely to beillustrative, rather than predictive, and variations in the testingprocedure can be expected to yield different results.

The present invention has now been described with reference to severalembodiments thereof. The foregoing detailed description and exampleshave been given for clarity of understanding only. No unnecessarylimitations are to be understood therefrom. All patents and patentapplications cited herein are hereby incorporated by reference. It willbe apparent to those skilled in the art that many changes can be made inthe embodiments described without departing from the scope of theinvention. Thus, the scope of the present invention should not belimited to the exact details and structures described herein, but ratherby the structures described by the language of the claims, and theequivalents of those structures.

1. In a method of stretching a polymeric film comprising the steps ofgrasping the film with a plurality of clips along the opposing edges ofthe film and propelling the clips to thereby stretch the film, whereinthe plurality of clips includes driven clips and idler clips, with atleast one idler clip between respective pairs of driven clips, theimprovement comprising: a) heating the polymeric film to a sufficientlyhigh temperature to allow a significant amount of stretching withoutbreaking; and b) imparting a machine direction cooling gradient to atleast a portion of the width of the stretched film in an effectiveamount to reduce the value of idler clip lag from the value of idlerclip lag in the absence of said cooling.
 2. The method of claim 1,wherein step b) includes actively cooling the opposed edge portions ofthe film.
 3. The method of claim 1, wherein step b) includes activelycooling the center portion of the film.
 4. The method of claim 1,wherein step b) includes actively cooling substantially the entire widthof the film.
 5. The method of claim 1, wherein step b) includes coolingat least a portion of the film by at least 3° C.
 6. The method of claim1, wherein the method further includes propelling the clips through astretch section in which the film is stretched and subsequently througha post-stretch treatment section, and wherein step b) is performed in atleast one of the stretch section and the treatment section.
 7. Themethod of claim 1, wherein the method includes biaxially stretching thefilm.
 8. The method of claim 1, wherein the method includessimultaneously biaxially stretching the film by propelling the clips atvarying speeds in the machine direction along clip guide means thatdiverge in the transverse direction.
 9. The method of claim 1, whereinthe film comprises polypropylene.
 10. The method of claim 9, wherein themethod includes stretching the film to a final area stretch ratio offrom 16:1 to 100:1.
 11. The method of claim 9, wherein step a) comprisesheating the film to from 120 to 165° C.
 12. The method of claim 11,wherein step b) includes forcing cooling air onto the film, wherein thecooling air is at least 5° C. cooler than the film.
 13. In a method ofstretching a polymeric film comprising the steps of grasping the filmwith a plurality of clips along the opposing edges of the film andpropelling the clips to thereby stretch the film, wherein the pluralityof clips includes driven clips and idler clips, with at least one idlerclip between respective pairs of driven clips, the improvementcomprising: a) heating the polymeric film to a sufficiently hightemperature to allow a significant amount of stretching withoutbreaking; and b) imparting a machine direction cooling gradient to atleast a portion of the width of the stretched film in an effectiveamount to improve the downweb caliper uniformity relative to the downwebcaliper uniformity in the absence of said cooling.
 14. The method ofclaim 13, wherein step b) includes actively cooling the opposed edgeportions of the film.
 15. The method of claim 13, wherein step b)includes actively cooling the center portion of the film.
 16. The methodof claim 13, wherein step b) includes actively cooling substantially theentire width of the film.
 17. The method of claim 13, wherein the methodfurther includes propelling the clips through a stretch section in whichthe film is stretched and subsequently through a post-stretch treatmentsection, and wherein step b) is performed in at least one of the stretchsection and the treatment section.
 18. The method of claim 13, whereinthe method includes simultaneously biaxially stretching the film bypropelling the clips at varying speeds in the machine direction alongclip guide means that diverge in the transverse direction.
 19. Themethod of claim 13, wherein the film comprises polypropylene.
 20. Themethod of claim 19, wherein the method includes stretching the film to afinal area stretch ratio of from 16:1 to 100:1.
 21. The method of claim19, wherein step a) comprises heating the film to from 120 to 165° C.22. The method of claim 19, wherein step b) includes forcing cooling aironto the film, wherein the cooling air is at least 5° C. cooler than thefilm.